This invention relates to an electronic device which is adapted to exhibit a function of automatically shutting off a power voltage, and more particularly to a power control circuit useful for a portable electronic device driven by a battery.
A portable or down-sized electronic device typically uses a rechargeable battery such as, for example, a nickel-cadmium battery or a conventional primary battery as a drive power supply therefor. Unfortunately, the battery fails to exhibit serviceability over an increased period of time because a power feed capacity thereof is substantially restricted.
In view of the foregoing, such a battery powered type electronic device as described above often includes a power control circuit which permits a power switch to be automatically turned off when an operation input is not carried out for a predetermined period of time, to thereby effectively prevent the battery from being subject to useless or wasteful power consumption.
Such a power control circuit is generally constructed in such a manner as shown in FIG. 3.
More particularly, the conventional power control circuit includes a battery or a secondary battery 10 which acts to feed an electronic device with a power supply voltage. The battery 10 is connected through a main switch 11 to various kinds of function circuits and control circuits to feed the circuits with an output of the battery.
Reference numeral 12 designates a power switch of the electronic device, which includes an on-contact a and an off-contact b which are selectively operated by means of a manually movable element. The on-contact a and off-contact b are connected through capacitors C1 and C2 to an S terminal of a flip-flop circuit 13 and an R terminal thereof, respectively, so that a Q output of the flip-flop circuit 13 is fed through an inverter IN to the above-described main switch 11, resulting in operation of the main switch 11 being carried out.
Reference numeral 14 designates a three-terminal voltage regulator, of which an output voltage causes the power supply voltage to be fed to a control circuit 15.
The control circuit 15 is provided with a logical circuit 16 for detecting cut-off conditions of the electronic device, so that, for example, when specific operation input is carried out over a predetermined period of time, the main switch 11 is automatically turned off in accordance with a judgment of the control circuit 15 which monitors an output of the logical circuit 15.
Thus, in the conventional power control circuit, when the movable element of the power supply switch 12 is moved to a side of the on-contact a as indicated by wave forms shown in FIG. 4, the S terminal of the flip-flop circuit 13 is fed with a negative trigger signal S.sub.1 through the capacitor C1 at time T.sub.1, so that the Q output of the flip-flop circuit 13 is inverted, resulting in the main switch 11 being turned on. This causes a power of the battery 10 to be fed through the thus turned on main switch 11 to the electronic device, resulting in the device being driven for various operations.
Also, when the movable element of the power switch 12 is changed over to a side of the off-contact b at time T.sub.2, a negative trigger signal S.sub.2 is fed through the capacitor C2 to the R contact of the flip-flop circuit 13, so that an inverted Q output of the flip-flop circuit 13 causes the main switch 11 to be turned off.
In FIG. 3, reference character R designates a resistor which comprises a pull-up resistor and R.sub.p is a resistor for setting a time constant of each of the trigger signals.
Then, when operation of the electronic equipment is not carried out over a predetermined period of time while the power supply is kept turned on, an auto cut-off signal generated from the logical circuit 16 detecting cut-off conditions is fed to the control circuit 15 at time T.sub.3, so that the control circuit 15 outputs a signal indicating an L level through the inverter IN. The signal thus output is subject to differentiation in the capacitor C3, so that a negative trigger signal S.sub.3 is fed to the R terminal of the flip-flop circuit 13.
Thus, it will be noted that the conventional power control circuit is so constructed that when non-operation of the electronic device continues over a predetermined period of time, the power supply is automatically turned off, to thereby eliminate useless power consumption, as well as wasteful power consumption due to a failure in or neglect of turning-off of the power switch 12 or the like.
Unfortunately, the conventional power control circuit causes a serious problem when it is applied to an electronic device which tends to cause significant malfunctions when a power supply voltage is shut off.
For example, when the conventional power control circuit is incorporated in an electronic device such as a radio control device which is adapted to operate a vehicle, an airplane or the like by radio control, environmental noise N generated in an operational environment shown in FIG. 4 such as, particularly, static electricity is introduced through a human body, an antenna or an operational switch into the electronic equipment. When the noise N is applied to the R terminal of the flip-flop circuit such a negative trigger signal S.sub.N as indicated at dotted lines in FIG. 4 causes the power supply to be turned off, resulting in the device being uncontrolled.